The α-glucosidase inhibitory substance inhibits α-glucosidase localized on the intestinal epithelium to suppress and delay decomposition/absorption of a saccharide. Thus, it has a suppressive action on elevation of blood sugar level. Therefore, the α-glucosidase inhibitory substance is useful for various diseases derived from chronic hyperglycemia, such as diabetes and adiposity. Since α-glucosidase inhibitory activity was found in a malt component in 1933, many α-glucosidase inhibitory substances derived from plants such as wheat and beans have been found.
In 1966, nojirimycin having α-glucosidase inhibitory activity was isolated from a microorganism metabolite and its structure was determined. An analogous compound thereof, 1-deoxynojirimycin, which was obtained from a mulberry-leaf extract, is known to have α-glucosidase inhibitory activity. A method for extracting it without reducing its activity is disclosed (Patent Literature 1).
Furthermore, it has been reported that a substance isolated from an extract of Kothalahimbutu's (Salacia reticulate) root and having a cyclitol structure of a 13-membered ring having a sulfoxide has high maltase inhibitory activity of 0.093 μM in terms of IC50 (Patent Literature 2).
Furthermore, as the anthocyanin compounds isolated from the roots of morning glory and violet sweet potato, namely, diacylated pelargonidin (IC50: 60-107 μM), cyanidin (IC50: 193 μM) and 3-sophoroside-5-glucoside (IC50: 200 μM) of peonidin, are reported to have maltase inhibitory activity (Non Patent Literature 1). Furthermore, substances such as theasinensin A (maltase inhibitory activity: 142 μM in terms of IC50) contained in tea leaves, a theaflavin derivative having a galloyl group and proanthocyanidin having an epiafzelechin gallate as a structural unit are confirmed to have maltase inhibitory activity(Patent Literature 3). However, a theaflavin derivative having a galloyl group has a maltase inhibitory activity of 10 to 136 μM in terms of IC50 but the content thereof in tea leaves is as extremely low as 0.1 to 0.2% (Non Patent Literature 2).
It has been reported that theaflavin of black tea and catechins of green tea have α-glucosidase inhibitory activity (Non Patent Literature 2). It is confirmed that (−)-epigallocatechin-3-O-gallate (hereinafter, also referred to as “EGCG”) and (−)-epicatechin-3-O-gallate having a galloyl group at position 3 among the catechins, and theaflavin-3-O-gallate and theaflavin-3,3′-di-O-gallate among the theaflavins have the activity. With respect to the α-glucosidase inhibitory activity of black tea, the activities of its fractionated products and the like have been investigated. A fraction of a polymer whose polymerization degree is advanced by fermentation is also known to have the activity (Non Patent Literature 3).
Citation List
Patent Literature
PTL 1: Japanese Patent Laid-Open No. 2007-60908
PTL 2: Japanese Patent Laid-Open No. 2008-137925
PTL 3: Japanese Patent Laid-Open No. 2007-231009
Non Patent Literature
NPL 1: J. Agric. Food Chem. 2001, 49, 1952-1956
NPL 2: J. Agric. Food. Chem., 55, 99-105, 2007
NPL 3: Chem. Pharm. Bull. 56(3), 266-272, 2008